Method of Forming Ga2O3-Based Crystal Film and Crystal Multilayer Structure

ABSTRACT

A method of forming a Ga 2 O 3 -based crystal film includes epitaxially growing a Ga 2 O 3 -based crystal film on a (001)-oriented principal surface of a Ga 2 O 3 -based substrate at a growth temperature of not less than 750° C. A crystal multilayer structure includes a Ga 2 O 3 -based substrate with a (001)-oriented principal surface, and a Ga 2 O 3 -based crystal film formed on the principal surface of the Ga 2 O 3 -based substrate by epitaxial growth. The principal surface has a flatness of not more than 1 nm in an RMS value.

The present application is based on Japanese patent application No.2013-265771 filed on Dec. 24, 2013, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of forming a Ga₂O₃-based crystal film,and a crystal multilayer structure including the Ga₂O₃-based crystalfilm.

2. Description of the Related Art

A technique is known in which a Ga₂O₃-based crystal film is epitaxiallygrown on a Ga₂O₃-based substrate (see e.g. WO 2013/035464).

Referring to WO 2013/035464, when the Ga₂O₃-based crystal film is grownon the Ga₂O₃-based substrate with a (001)-oriented principal surface ata growth temperature of 700° C., the growth rate of the Ga₂O₃-basedcrystal film is about 90 nm/h. Likewise, when the Ga₂O₃-based crystalfilm is grown on the Ga₂O₃-based substrate with a (010)-orientedprincipal surface at a growth temperature of 700° C., the growth rate ofthe Ga₂O₃-based crystal film is about 130 nm/h.

SUMMARY OF THE INVENTION

The growth rate of the Ga₂O₃-based crystal film needs to be higher,e.g., about 0.1 μm/h at the minimum, in terms of the mass productivityof the Ga₂O₃-based crystal film. However, even if the growth rate issufficiently increased, the Ga₂O₃-based crystal film with a poorcrystalline quality or insufficient surface flatness may not bepractically used.

It is an object of the invention to provide a method of forming aGa₂O₃-based crystal film that allows a Ga₂O₃-based crystal film with anexcellent crystal quality and surface flatness to be formed at a highgrowth rate suitable for the mass productivity, as well as a crystalmultilayer structure including the Ga₂O₃-based crystal film formed bythe method.

According to one embodiment of the invention, a method of forming aGa2O3-based crystal film as defined in [1] to [3] below is provided.

[1] A method of forming a Ga₂O₃-based crystal film, comprisingepitaxially growing a Ga₂O₃-based crystal film on a (001)-orientedprincipal surface of a Ga₂O₃-based substrate at a growth temperature ofnot less than 750° C.

[2] The method according to [1], wherein the principal surface of theGa₂O₃-based crystal film has a flatness of not more than 1 nm in an RMSvalue.

[3] The method according to [1] or [2], wherein the Ga₂O₃-based crystalfilm comprises a Ga₂O₃ crystal film.

According to another embodiment of the invention, a crystal multilayerstructure as defined in [4] or [5] below is provided.

[4] A crystal multilayer structure, comprising:

a Ga₂O₃-based substrate with a (001)-oriented principal surface; and

a Ga₂O₃-based crystal film formed on the principal surface of theGa₂O₃-based substrate by epitaxial growth,

wherein the principal surface has a flatness of not more than 1 nm in anRMS value.

[5] The crystal multilayer structure according to [4], wherein theGa₂O₃-based crystal film comprises a Ga₂O₃ crystal film.

Effects of the Invention

According to one embodiment of the invention, a method of forming aGa₂O₃-based crystal film can be provided that allows a Ga₂O₃-basedcrystal film with an excellent crystal quality and surface flatness tobe formed at a high growth rate suitable for the mass productivity, aswell as a crystal multilayer structure including the Ga₂O₃-based crystalfilm formed by the method.

BRIEF DESCRIPTION OF THE DRAWINGS

Next, the present invention will be explained in more detail inconjunction with appended drawings, wherein:

FIG. 1 is a vertical cross-sectional view showing a crystal multilayerstructure in a first embodiment;

FIGS. 2A and 2B are graphs showing a relation between a growthtemperature and flatness of a principal surface of a Ga₂O₃ crystal filmwhen epitaxially grown on a (010)-oriented principal surface of a Ga₂O₃substrate and a (−201)-oriented principal surface of a Ga₂O₃ substrate;

FIGS. 3A and 3B are graphs showing a relation between a growthtemperature and flatness of a principal surface of a Ga₂O₃ crystal filmwhen epitaxially grown on a (101)-oriented principal surface of a Ga₂O₃substrate and a (001)-oriented principal surface of a Ga₂O₃ substrate;

FIG. 4 is a graph showing a relation between growth temperature andgrowth rate of the Ga₂O₃ crystal film when epitaxially grown on the(001)-oriented principal surface of the Ga₂O₃ substrate;

FIG. 5 is a graph showing X-ray diffraction spectra obtained by X-rayrocking curve measurement on crystal multilayer structures in which aGa₂O₃ crystal film is epitaxially grown on the (001)-oriented principalsurface of the Ga₂O₃ substrate;

FIG. 6 is a graph showing X-ray diffraction spectra obtained by 2θ-ωscan on crystal multilayer structures in which a Ga₂O₃ crystal film isepitaxially grown on the (001)-oriented principal surface of the Ga₂O₃substrate;

FIG. 7 is a vertical cross-sectional view showing ahigh-electron-mobility transistor in a second embodiment;

FIG. 8 is a vertical cross-sectional view showing a MESFET in a thirdembodiment;

FIG. 9 is a vertical cross-sectional view showing a Schottky-barrierdiode in a fourth embodiment; and

FIG. 10 is a vertical cross-sectional view showing a MOSFET in a fifthembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a vertical cross-sectional view showing a crystal multilayerstructure in the first embodiment. A crystal multilayer structure 1 hasa Ga₂O₃-based substrate 10 and a Ga₂O₃-based crystal film 12 formed onthe Ga₂O₃-based substrate 10 by epitaxial crystal growth.

The Ga₂O₃-based substrate 10 is a substrate formed of a Ga₂O₃-basedsingle crystal. The Ga₂O₃-based single crystal here is referred to aGa₂O₃ single crystal or a Ga₂O₃ single crystal doped with an elementsuch as Al and In, and may be, e.g., a (Ga_(x)Al_(y)In_((1-x-y)))₂O₃(0<x≦1, 0≦y<1, 0<x+y≦1) single crystal which is a Ga₂O₃ single crystaldoped with Al and In. The band gap is widened by adding Al and isnarrowed by adding In. The Ga₂O₃ single crystal mentioned above has,e.g., a β-crystal structure. The Ga₂O₃-based substrate 10 may contain aconductive impurity such as Si.

The orientation of a principal surface 11 of the Ga₂O₃-based substrate10 is (001).

For forming the Ga₂O₃-based substrate 10, for example, a bulk crystal ofa Ga₂O₃-based single crystal is grown by melt-growth method such as FZ(Floating Zone) method or EFG (Edge Defined Film Fed Growth) method, issliced and is then surface-polished.

The Ga₂O₃-based crystal film 12 is formed of a Ga₂O₃-based singlecrystal, similarly to the Ga₂O₃-based substrate 10. Since theGa₂O₃-based crystal film 12 is formed on the principal surface 11 of theGa₂O₃-based substrate 10 by epitaxial crystal growth, the orientation ofa principal surface 13 of the Ga₂O₃-based crystal film 12 is also (001)in the same manner as the principal surface 11 of the Ga₂O₃-basedsubstrate 10. The Ga₂O₃-based crystal film 12 may contain a conductiveimpurity such as Si.

The Ga₂O₃-based crystal film 12 is formed by, e.g., physical vapordeposition such as MBE (Molecular Beam Epitaxy) method or chemical vapordeposition such as CVD (Chemical Vapor Deposition) method.

The Ga₂O₃-based crystal film 12 is formed by epitaxial growth at agrowth temperature of not less than 750° C. Flatness of the principalsurface 13 of the Ga₂O₃-based crystal film 12 is improved by growing ata growth temperature of not less than 750° C. In detail, the RMS valueof the principal surface 13 is not more than 1 nm.

Here, the RMS value is a numerical value to be an index of flatness, andis obtained by measuring a curve representing a relation betweenvertical height and horizontal position of the principal surface of theGa₂O₃ crystal film using an atomic force microscope and then calculatingthe square root of the means of the squares of the deviation from theaverage line to the curve.

When manufacturing, e.g., Schottky diode or MESFET (Metal-SemiconductorField Effect Transistor) using the crystal multilayer structure 1 with alarge RMS value, there is a possibility that electric fieldconcentration occurs at a Schottky electrode formed on the Ga₂O₃-basedcrystal film 12 and causes a decrease in element withstand voltage. Thisis caused because an electric field is concentrated at raised portionsof irregularities on a bottom surface of the Schottky electrode which isformed due to irregularities on the principal surface 13 of theGa₂O₃-based crystal film 12. That the RMS value is not more than 1 nm isknown as a surface roughness condition for the bottom surface of theSchottky electrode to suppress the electric field concentration. Inother words, it is possible to suppress the electric field concentrationat the Schottky electrode when the RMS value of the principal surface 13of the Ga₂O₃-based crystal film 12 is not more than 1 nm.

In addition, the Ga₂O₃-based crystal film 12 with high crystal qualityis obtained by growing at a growth temperature of not less than 750° C.If growth temperature is more than 900° C., re-evaporation of thesupplied Ga drastically proceeds and the growth rate drops to 1/10 orless of the growth rate at a growth temperature of 600° C. As such,lower growth temperature is preferable from the viewpoint of efficiencyof raw material consumption. Therefore, growth temperature of theGa₂O₃-based crystal film 12 is preferably not more than 900° C.

Since the Ga₂O₃-based crystal film 12 is excellent in crystal qualityand flatness of principal surface, it is possible to form high-qualitymetal-semiconductor interface and insulating film-semiconductorinterface on the Ga₂O₃-based crystal film 12. Therefore, it is possibleto manufacture high-quality semiconductor devices by using the crystalmultilayer structure 1.

Evaluation of Ga₂O₃-Based Crystal Film

Results of evaluating flatness of principal surface, a growth rate andcrystal quality of the Ga₂O₃-based crystal film are described below. Forthe evaluation, a Ga₂O₃ substrate was used as the Ga₂O₃-based substrateand a Ga₂O₃ crystal film having a thickness of about 100 to 300 nm wasformed as the Ga₂O₃-based crystal film by the MBE method. Ozone was usedas an oxygen source for the Ga₂O₃ crystal film.

FIGS. 2A and 2B are graphs showing a relation between a growthtemperature and flatness of a principal surface of a Ga₂O₃ crystal filmwhen epitaxially grown on a (010)-oriented principal surface of a Ga₂O₃substrate and a (−201)-oriented principal surface of a Ga₂O₃ substrate.

FIGS. 3A and 3B are graphs showing a relation between a growthtemperature and flatness of a principal surface of a Ga₂O₃ crystal filmwhen epitaxially grown on a (101)-oriented principal surface of a Ga₂O₃substrate and a (001)-oriented principal surface of a Ga₂O₃ substrate.

In FIGS. 2A, 2B, 3A and 3B, the horizontal axis indicates growthtemperature (° C.) of a Ga₂O₃ crystal film and the vertical axisindicates an RMS value (nm) of the principal surface of the Ga₂O₃crystal film. The RMS value was calculated on an atomic force microscopeimage within an area of 1 μm square.

FIGS. 2B and 3A show that, when the orientation of the principal surfaceof the Ga₂O₃ substrate is (−201) or (101), a Ga₂O₃ crystal filmexcellent in flatness (e.g., the principal surface with the RMS value ofnot more than 1 nm) is not obtained regardless of growth temperature. AGa₂O₃ crystal film with low flatness could be polished to improveflatness but this increases the number of processes and causes aresulting increase in the manufacturing cost, hence, it is notpreferable.

Meanwhile, FIG. 2A shows that, when the orientation of the principalsurface of the Ga₂O₃ substrate is (010), a Ga₂O₃ crystal film having theprincipal surface with the RMS value of not more than 1 nm is obtainedat a growth temperature of about 550 to 650° C. However, it is notpossible to obtain a Ga₂O₃ crystal film with high crystal quality at agrowth temperature of about 550 to 650° C. In detail, for example, anetch pit density of the Ga₂O₃ crystal film is about 10⁶ cm⁻² whenforming at a growth temperature of 600° C. but decreases to about 10⁴cm⁻² when forming at a growth temperature of 700° C. (i.e., defectsdecrease to 1/100). A Ga₂O₃ crystal film with quality equivalent to theGa₂O₃ substrate, of which the etch pit density is about 10⁴ cm⁻², isobtained by setting the growth temperature to not less than 700° C.

However, as seen in FIG. 2A, it is not possible to obtain a Ga₂O₃crystal film having a principal surface with the RMS value of not morethan 1 nm at not less than 700° C. This shows that it is difficult toobtain a Ga₂O₃ crystal film excellent in both crystal quality andflatness of principal surface when the orientation of the principalsurface of the Ga₂O₃ substrate is (010).

Meanwhile, FIG. 3B shows that, when the orientation of the principalsurface of the Ga₂O₃ substrate is (001), a Ga₂O₃ crystal film having theprincipal surface with the RMS value of not more than 1 nm is obtainedat a growth temperature of not less than 750° C.

Similar evaluation results to the above are obtained both when anotherGa₂O₃-based substrate is used in place of the Ga₂O₃ substrate and whenanother Ga₂O₃-based crystal film is formed instead of forming the Ga₂O₃crystal film. In other words, when the orientation of the principalsurface of the Ga₂O₃-based substrate is (001), a Ga₂O₃-based crystalfilm having the principal surface with the RMS value of not more than 1nm is obtained by growing at a growth temperature of not less than 750°C.

FIG. 4 is a graph showing a relation between growth temperature andgrowth rate of the Ga₂O₃ crystal film when epitaxially grown on the(001)-oriented principal surface of the Ga₂O₃ substrate.

In FIG. 4, the horizontal axis indicates growth temperature (° C.) ofthe Ga₂O₃ crystal film and the vertical axis indicates a growth rate(μm/h) of the Ga₂O₃ crystal film.

It is understood from FIG. 4 that a growth rate at a growth temperatureof not less than 750° C., at which the Ga₂O₃ crystal film having theprincipal surface with the RMS value of not more than 1 nm is obtained,is lower than the growth rate at a relatively row growth temperature of600 to 700° C. but is about 0.4 μm/h which does not cause any problemfor mass production of the Ga₂O₃ crystal film.

Similar evaluation results to the above are obtained both when anotherGa₂O₃-based substrate is used in place of the Ga₂O₃ substrate and whenanother Ga₂O₃-based crystal film is formed instead of forming the Ga₂O₃crystal film. In other words, when the orientation of the principalsurface of the Ga₂O₃-based substrate is (001), a growth rate of about0.4 μm/h is obtained at a growth temperature of not less than 750° C.

FIG. 5 is a graph showing X-ray diffraction spectra obtained by X-rayrocking curve measurement on crystal multilayer structures in which aGa₂O₃ crystal film is epitaxially grown on the (001)-oriented principalsurface of the Ga₂O₃ substrate.

In FIG. 5, the horizontal axis indicates an incidence angle ω (degree)of an x-ray and the vertical axis indicates diffraction intensity(arbitrary unit) of the x-ray.

FIG. 5 shows a spectrum of the Ga₂O₃ substrate (without Ga₂O₃ crystalfilm) and spectra of crystal multilayer structures in which the Ga₂O₃crystal film is epitaxially grown respectively at 600° C., 650° C., 700°C., 725° C., 750° C. and 775° C. In FIG. 5, a diffraction peak in eachspectrum is a diffraction peak of (002) plane.

FIG. 5 shows that the diffraction peak of the any of the crystalmultilayer structures has substantially the same half width as thediffraction peak of the Ga₂O₃ substrate. This shows that a Ga₂O₃ crystalfilm with small variation in a crystal orientation is obtained bygrowing at any growth temperature between 600 to 775° C.

Similar evaluation results to the above are obtained both when anotherGa₂O₃-based substrate is used in place of the Ga₂O₃ substrate and whenanother Ga₂O₃-based crystal film is formed instead of forming the Ga₂O₃crystal film. In other words, when the orientation of the principalsurface of the Ga₂O₃-based substrate is (001), it is possible to obtaina Ga₂O₃-based crystal film with small variation in a crystal orientationby growing at any growth temperature between 600 to 775° C.

FIG. 6 is a graph showing X-ray diffraction spectra obtained by 2θ-ωscan on crystal multilayer structures in which a Ga₂O₃ crystal film isepitaxially grown on the (001)-oriented principal surface of the Ga₂O₃substrate.

In FIG. 6, the horizontal axis indicates an angle 2θ (degree) formedbetween incidence and reflection directions of an x-ray and the verticalaxis indicates diffraction intensity (arbitrary unit) of the x-ray.

FIG. 6 shows a spectrum of the Ga₂O₃ substrate (without Ga₂O₃ crystalfilm) and spectra of crystal multilayer structures in which the Ga₂O₃crystal film is epitaxially grown respectively at 600° C., 650° C., 700°C., 725° C., 750° C. and 775° C.

FIG. 6 shows that a diffraction peak of (−401) plane, which occurs dueto presence of heterogeneous phase and is observed in spectra of thecrystal multilayer structures having a Ga₂O₃ crystal film grown at agrowth temperature between 600 to 725° C., disappears in spectra of thecrystal multilayer structures having a Ga₂O₃ crystal film grown at agrowth temperature of not less than 750° C. This shows that asingle-phase Ga₂O₃ crystal film is obtained by growing at a growthtemperature of not less than 750° C. The broad peak around 2θ=26° occursdue to diffraction from a substrate holder of an X-ray diffractometer.

In addition, the etch pit density of the Ga₂O₃ crystal film formed at agrowth temperature of 750° C. is about 10⁴ cm⁻² and is substantiallyequal to the etch pit density of the Ga₂O₃ substrate which is also about10⁴ cm⁻². This shows that the Ga₂O₃ crystal film has high crystalquality equivalent to the Ga₂O₃ substrate.

Similar evaluation results to the above are obtained both when anotherGa₂O₃-based substrate is used in place of the Ga₂O₃ substrate and whenanother Ga₂O₃-based crystal film is formed instead of forming the Ga₂O₃crystal film. In other words, when the orientation of the principalsurface of the Ga₂O₃-based substrate is (001), a single-phaseGa₂O₃-based crystal film is obtained by growing at a growth temperatureof not less than 750° C.

Considering the evaluation results obtained from the X-ray diffractionspectra in FIG. 5 together with the evaluation results obtained from theX-ray diffraction spectra in FIG. 6, it is understood that a Ga₂O₃-basedcrystal film excellent in crystal quality is obtained by growing at agrowth temperature of not less than 750° C.

Second Embodiment

A high-electron-mobility transistor (HEMT), which is one ofsemiconductor devices including the Ga₂O₃-based substrate 10 and theGa₂O₃-based crystal film 12 of the first embodiment, will be describedas the second embodiment.

FIG. 7 is a vertical cross-sectional view showing ahigh-electron-mobility transistor in the second embodiment. Ahigh-electron-mobility transistor 2 includes the Ga₂O₃-based substrate10 and the Ga₂O₃-based crystal film 12 of the first embodiment. Thehigh-electron-mobility transistor 2 further includes an electron supplylayer 21 on the principal surface 13 of the Ga₂O₃-based crystal film 12,and a gate electrode 23, a source electrode 24 and a drain electrode 25which are provided on the electron supply layer 21. The gate electrode23 is arranged between the source electrode 24 and the drain electrode25.

The gate electrode 23 is in contact with a principal surface 22 of theelectron supply layer 21, thereby forming a Schottky junction.Meanwhile, the source electrode 24 and the drain electrode 25 are incontact with the principal surface 22 of the electron supply layer 21,thereby forming an ohmic junction.

In the second embodiment, the Ga₂O₃-based substrate 10 contains group IIelements such as Mg and has high electrical resistance. Meanwhile, theGa₂O₃-based crystal film 12 is of an i-type and functions as an electrontransit layer.

The electron supply layer 21 is formed of, e.g., a β-(AlGa)₂O₃ singlecrystal doped with a donor such as Si or Sn and is formed by epitaxialgrowth on the Ga₂O₃-based crystal film 12.

Since the Ga₂O₃-based crystal film 12 and the electron supply layer 21have different band gaps, discontinuity of bands occurs at the interfacetherebetween, electrons generated from the donor in the electron supplylayer 21 are concentrated on the Ga₂O₃-based crystal film 12 side andare distributed in a region in the vicinity of the interface, and anelectron layer called two-dimensional electron gas is thereby formed.

As such, a first depletion layer due to the Schottky junction with thegate electrode 23 and a second depletion layer due to the formation oftwo-dimensional electron gas are produced in the electron supply layer21. The electron supply layer 21 has such a thickness that the firstdepletion layer is in contact with the second depletion layer.

Voltage is applied to the gate electrode 23 to change the thicknesses ofthe first and second depletion layers and to adjust the concentration ofthe two-dimensional electron gas, thereby controlling a drain current.

The thickness of the Ga₂O₃-based crystal film 12 is not specificallylimited but is preferably not less than 1 nm. In addition, the thicknessof the electron supply layer 21 is set between 0.001 and 1 μm dependingon a doping concentration.

In the high-electron-mobility transistor 2, since flatness of theprincipal surface 13 of the Ga₂O₃-based crystal film 12 is high,flatness of the principal surface 22 of the electron supply layer 21formed on the Ga₂O₃-based crystal film 12 is also high and electricfield concentration at the gate electrode 23 forming, together withelectron supply layer 21, a Schottky junction is suppressed. Therefore,a decrease in withstand voltage performance of thehigh-electron-mobility transistor 2 is suppressed.

Third Embodiment

A MESFET (Metal-Semiconductor Field Effect Transistor), which is one ofsemiconductor devices including the Ga₂O₃-based substrate 10 and theGa₂O₃-based crystal film 12 of the first embodiment, will be describedas the third embodiment.

FIG. 8 is a vertical cross-sectional view showing a MESFET in the thirdembodiment. A MESFET 3 includes the Ga₂O₃-based substrate 10 and theGa₂O₃-based crystal film 12 of the first embodiment. The MESFET 3further includes a gate electrode 31, a source electrode 32 and a drainelectrode 33 which are provided on the Ga₂O₃-based crystal film 12. Thegate electrode 31 is arranged between the source electrode 32 and thedrain electrode 33.

The gate electrode 31 is in contact with the principal surface 13 of theGa₂O₃-based crystal film 12, thereby forming a Schottky junction.Meanwhile, the source electrode 32 and the drain electrode 33 are incontact with the principal surface 13 of the Ga₂O₃-based crystal film12, thereby forming an ohmic junction.

In the third embodiment, the Ga₂O₃-based substrate 10 contains group IIelements such as Mg and has high electrical resistance.

In the third embodiment, the Ga₂O₃-based crystal film 12 is of an n-typeand the donor concentration thereof in the vicinity of contact areaswith the source electrode 32 and with the drain electrode 33 is higherthan that in the remaining portion.

The thickness of the depletion layer in the Ga₂O₃-based crystal film 12under the gate electrode 31 is changed by controlling bias voltageapplied to the gate electrode 31, thereby allowing a drain current to becontrolled.

In the MESFET 3, since flatness of the principal surface 13 of theGa₂O₃-based crystal film 12 is high, electric field concentration at thegate electrode 31 forming, together with the Ga₂O₃-based crystal film12, a Schottky junction is suppressed. Therefore, a decrease inwithstand voltage performance of the MESFET 3 is suppressed.

Fourth Embodiment

A Schottky-barrier diode, which is one of semiconductor devicesincluding the Ga₂O₃-based substrate 10 and the Ga₂O₃-based crystal film12 of the first embodiment, will be described as the fourth embodiment.

FIG. 9 is a vertical cross-sectional view showing a Schottky-barrierdiode in the fourth embodiment. A Schottky-barrier diode 4 includes theGa₂O₃-based substrate 10 and the Ga₂O₃-based crystal film 12 of thefirst embodiment. The Schottky-barrier diode 4 further includes aSchottky electrode 41 on the principal surface 13 of the Ga₂O₃-basedcrystal film 12 and an ohmic electrode 42 on a principal surface 14 ofthe Ga₂O₃-based substrate 10 opposite to the principal surface 11.

The Schottky electrode 41 is in contact with the principal surface 13 ofthe Ga₂O₃-based crystal film 12, thereby forming a Schottky junction.Meanwhile, the ohmic electrode 42 is in contact with the principalsurface 14 of the Ga₂O₃-based substrate 10, thereby forming an ohmicjunction.

In the fourth embodiment, the Ga₂O₃-based substrate 10 and theGa₂O₃-based crystal film 12 are of an n-type and the donor concentrationof the Ga₂O₃-based crystal film 12 is lower than that of the Ga₂O₃-basedsubstrate 10.

When forward voltage (electric potential is positive on the Schottkyelectrode 41 side) is applied to the Schottky-barrier diode 4, thenumber of electrons moving from the Ga₂O₃-based substrate 10 to theGa₂O₃-based crystal film 12 is increased. As a result, a forward currentflows from the Schottky electrode 41 to the ohmic electrode 42.

On the other hand, when reverse voltage (electric potential is negativeon the Schottky electrode 41 side) is applied to the Schottky-barrierdiode 4, substantially no electric current flows through theSchottky-barrier diode 4.

In the Schottky-barrier diode 4, since flatness of the principal surface13 of the Ga₂O₃-based crystal film 12 is high, electric fieldconcentration at the Schottky electrode 41 forming, together with theGa₂O₃-based crystal film 12, a Schottky junction is suppressed.Therefore, a decrease in withstand voltage performance of theSchottky-barrier diode 4 is suppressed.

Fifth Embodiment

A MOSFET (Metal-oxide-Semiconductor Field Effect Transistor), which isone of semiconductor devices including the Ga₂O₃-based substrate 10 andthe Ga₂O₃-based crystal film 12 of the first embodiment, will bedescribed as the fifth embodiment.

FIG. 10 is a vertical cross-sectional view showing a MOSFET in the fifthembodiment. A MOSFET 5 includes the Ga₂O₃-based substrate 10 and theGa₂O₃-based crystal film 12 of the first embodiment. The MOSFET 5further includes an oxide insulating film 52, a gate electrode 51, asource electrode 53 and a drain electrode 54 which are provided on theGa₂O₃-based crystal film 12. The insulating film 52 and the gateelectrode 51 are arranged between the source electrode 53 and the drainelectrode 54.

The gate electrode 51 is formed on the principal surface 13 of theGa₂O₃-based crystal film 12 via the insulating film 52. Meanwhile, thesource electrode 53 and the drain electrode 54 are in contact with theprincipal surface 13 of the Ga₂O₃-based crystal film 12, thereby formingan ohmic junction.

In the fifth embodiment, the Ga₂O₃-based substrate 10 contains group IIelements such as Mg and has high electrical resistance.

It is possible to control a drain current flowing through theGa₂O₃-based crystal film 12 by controlling bias voltage applied to thegate electrode 51.

In the MOSFET 5, since flatness of the principal surface 13 of theGa₂O₃-based crystal film 12 is high, it is possible to provide a steepinterface between the Ga₂O₃-based crystal film 12 and the oxideinsulating film 52 and thereby to suppress electric field concentrationor a decrease in channel mobility, etc.

Effects of the Embodiments

According to the embodiments, it is possible to form a Ga₂O₃-basedcrystal film with excellent crystal quality and principal surfaceflatness at a growth rate high enough for mass production. In addition,since the Ga₂O₃-based crystal film is excellent in crystal quality andflatness of principal surface, it is possible to grow a good-qualitycrystal film on the Ga₂O₃-based crystal film. Thus, use of the crystalmultilayer structure including the Ga₂O₃-based crystal film in theembodiment allows high-quality semiconductor devices to be manufactured.

Although the embodiments of the invention have been described, theinvention is not intended to be limited to these embodiments, and thevarious kinds of modifications can be implemented without departing fromthe gist of the invention.

In addition, the invention according to claims is not to be limited tothe embodiments. Further, it should be noted that all combinations ofthe features described in the embodiments are not necessary to solve theproblem of the invention.

What is claimed is:
 1. A method of forming a Ga₂O₃-based crystal film,comprising epitaxially growing a Ga₂O₃-based crystal film on a(001)-oriented principal surface of a Ga₂O₃-based substrate at a growthtemperature of not less than 750° C.
 2. The method according to claim 1,wherein the principal surface of the Ga₂O₃-based crystal film has aflatness of not more than 1 nm in an RMS value.
 3. The method accordingto claim 1, wherein the Ga₂O₃-based crystal film comprises a Ga₂O₃crystal film.
 4. The method according to claim 2, wherein theGa₂O₃-based crystal film comprises a Ga₂O₃ crystal film.
 5. A crystalmultilayer structure, comprising: a Ga₂O₃-based substrate with a(001)-oriented principal surface; and a Ga₂O₃-based crystal film formedon the principal surface of the Ga₂O₃-based substrate by epitaxialgrowth, wherein the principal surface has a flatness of not more than 1nm in an RMS value.
 6. The crystal multilayer structure according toclaim 5, wherein the Ga₂O₃-based crystal film comprises a Ga₂O₃ crystalfilm.